1. Field of the Invention
The present invention relates to a state determining apparatus for determining the state of an exhaust gas recirculation system which includes an EGR passage for recirculating a portion of exhaust gases to an intake system in accordance with an operating state of an internal combustion engine.
2. Description of the Prior Art
A conventional state determining apparatus of the type mentioned above is known, for example, in Laid-open Japanese Patent Application No. 63-243446. This known state determining apparatus is provided for determining a failure of an EGR control valve for controlling an ERG passage to open and close, and comprises a temperature sensor at a location downstream of the EGR control valve in the EGR passage for detecting the temperature of an EGR gas. Then, if a detected EGR gas temperature is lower than a first predetermined temperature when the EGR control valve is supplied with a signal for opening the EGR passage, the state determining apparatus determines that the EGR control valve has failed due to a stick or the like because no EGR gas is actually flowing through the EGR passage. On the other hand, if a detected EGR gas temperature is higher than a second predetermined temperature lower than the first predetermined temperature when the EGR control valve is supplied with a signal for closing the EGR passage, the state determining apparatus determines that the EGR control valve has failed because the EGR gas is actually passing through the EGR passage.
As described above, the conventional failure determining apparatus relies on the EGR gas temperature detected by the temperature sensor provided in the EGR passage as a parameter for determining a failure of the EGR control valve. However, characteristically, the xe2x80x9ctemperaturexe2x80x9d is inherently slow in change, low in responsibility, and susceptible to a variety of factors such as an external air temperature and the like. With such characteristics of the temperature, the failure determining apparatus may fail to appropriately determine a failure of the EGR control valve, and would suffer from a lower accuracy of determination when it is applied to the determination of a leak on the EGR passage or the like, due to a large length of the EGR passage which causes the characteristics of the temperature to prominently appear.
The present invention has been made to solve the problem mentioned above, and it is an object of the invention to provide a state determining apparatus for an exhaust gas recirculation system which is capable of appropriately determining the state of an exhaust gas recirculation system including an EGR passage.
To achieve the above object, the present invention provides an exhaust gas recirculation system state determination apparatus for determining a state of an exhaust gas recirculation system arranged in an exhaust system of an internal combustion engine including an EGR passage for recirculating a portion of exhaust gases to an intake system in accordance with an operating state of the internal combustion engine, characterized by comprising a humidity sensor arranged in the EGR passage for detecting a humidity within the EGR passage; and exhaust gas recirculation system state determining means for determining the state of the exhaust gas recirculation system based on a result detected by the humidity sensor.
According to this state determining apparatus, the humidity of exhaust gases in the EGR passage (hereinafter called the xe2x80x9cEGR gasxe2x80x9d) is detected by the humidity sensor arranged in the EGR passage, and the state of the exhaust gas recirculation system including the EGR passage is determined by the exhaust gas recirculation system state determining means based on the result of detection. Since the EGR gas is a portion of exhaust gases generated by combustion, it generally includes a large amount of moisture, and therefore the humidity reflects the state of the exhaust gas recirculation system including the EGR passage through which the EGR gas passes. It is therefore possible to appropriately determine the state of the exhaust gas recirculation system including the EGR passage, for example, a failure due to a leak and the like in the EGR passage based on the result of detection made by the humidity sensor. Also, since the humidity is characteristically high in responsibility and less susceptible to other factors, as compared with the temperature, a higher determination accuracy can be achieved than when the temperature is used as a parameter.
Preferably, in the state determining apparatus, the exhaust system includes a main passage, a bypass passage branched off the main passage, an adsorbent arranged in the bypass passage and capable of adsorbing hydrocarbons and moisture, and switching means for switching an exhaust passage to the main passage and to the bypass passage, wherein the exhaust passage is switched to the bypass passage by the switching means after the internal combustion engine has been started to adsorb hydrocarbons within exhaust gases on the adsorbent, and the adsorbent hydrocarbons are desorbed and recirculated to the intake system through the EGR passage.
According to this preferred embodiment, the EGR passage is used as a passage for recirculating hydrocarbons desorbed from the adsorbent, on which they have once been adsorbed, to the intake system. Also, since the adsorbent is capable of adsorbing moisture as well as hydrocarbons, there is a mutually close relationship between hydrocarbons and moisture with respect to their adsorption and desorption states, and they have a high correlation. Stated another way, the humidity of the EGR gas reflects not only the state in the EGR passage but also the state of the adsorbent during desorption. It is therefore possible to appropriately determine based on the result of detection made by the humidity sensor the state of the adsorbent, for example, whether or not hydrocarbons have been completely desorbed from the adsorbent.
In a general EGR device, a majority of EGR passage is formed in a cylinder block of an engine, and the EGR passage has a relatively short length around the engine, so that a leak from the EGR passage does not cause a grave problem. On the other hand, an EGR passage containing an adsorbent as described above is often formed as a relatively long EGR pipe for connecting the downstream side of the adsorbent to an intake system, so that the EGR pipe is more susceptible to a leak due to cracking and the like possibly caused by vibrations of the engine and the difference in thermal expansion in the exhaust system. According to this preferred embodiment, it is possible to efficiently determine a leak from the EGR passage and the state of the adsorbent in such a situation based on the result of detection made by the common humidity sensor.
Preferably, in the state determining apparatus, the exhaust gas recirculation system state determining means includes humidity integrated value calculating means for calculating an integrated value of the result detected by the humidity sensor during desorption of hydrocarbons from the adsorbent; and
desorption state determining means for determining a desorption state of the adsorbent based on the integrated value calculated by the humidity integrated value calculating means.
As described above with respect to the preferred embodiment, the result of detection made by the humidity sensor during the desorption of hydrocarbons from the adsorbent precisely reflects the amount of desorbed hydrocarbons, and has a high correlation to this. It is therefore possible to appropriately determine a desorption state of the adsorbent, for example, the completion of adsorption based on the calculation of the humidity integrated value which is an integrated value of the result of detection made by the humidity sensor.